Error in rotation 13 cipher code in python

Question

I was doing some competitive programming and stumbled upon a ROT13 question that required me to increment each alphabet by 13.

This was my attempt

def rot13(message):
    l2 = []
    l1 = list(message)
    for i in l1:
        i = str(i)
    for i in l1:
        if ord('a') <= ord(i) <= ord('z'):
            i = chr((ord(i) + 13) % 26 + ord('a'))
            l2.append(i)
        elif ord('A') <= ord(i) <= ord('Z'):
            i = chr((ord(i) + 13) % 26 + ord('A'))
            l2.append(i)
    return l2

It was returning wrong outputs such as

For input - test , it gave output - zkyz while correct is "grfg"

For input - Test , output was Tkyz while it should be Grfg

I haven't joined the list yet as I was first trying to get the right answer.

You forget to subtract the initial value from the characters so 'A' starts out as 65, not 0. — Jongware
– Jongware, Commented Jan 27, 2020 at 7:32
@shawnin damnen If you are looking for a reference take a look at the this module implementation. — user459872
– user459872, Commented Jan 27, 2020 at 7:33
@usr2564301 Which is why I have added it at the end to give it the unicode integer value — shawnin damnen
– shawnin damnen, Commented Jan 27, 2020 at 7:34

Raymond Hettinger · Accepted Answer · 2020-01-27 08:49:33Z

2

Here's the corrected code:

def rot13(message):
    l2 = []
    l1 = list(message)
    for i in l1:
        if ord('a') <= ord(i) <= ord('z'):
            i = chr((ord(i) - ord('a') + 13) % 26 + ord('a'))  # <== changed
            l2.append(i)
        elif ord('A') <= ord(i) <= ord('Z'):
            i = chr((ord(i) - ord('A') + 13) % 26 + ord('A'))  # <== changed
            l2.append(i)
    return l2

and a test run:

>>> rot13('Test')
['G', 'r', 'f', 'g']

The issue was that ord('a') or ord('A') needed to be subtracted from the initial ord() call. You were pretty close. Except for this nit, it all worked :-)

edited Jan 27, 2020 at 8:49

answered Jan 27, 2020 at 7:45

Raymond Hettinger

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4 Comments

Raymond Hettinger Over a year ago

@shawnindamnen Please mark the answer as the selected answer. Thx.

FredrikHedman Over a year ago

Why the first for-loop?

Raymond Hettinger Over a year ago

@FredrikHedman I just took that out. It was there because it was in the OP's code and I wanted to make the minimal edits required to make it work. But, yes, that loop was likely superfluous.

FredrikHedman Over a year ago

Yes. However, this solution will drop any charachter that is not [a-zA-Z]. For example rot13('Smörgåsbord').

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